Some cables that can be laid underwater have copper wires, or conductors, covered with an insulator made of polymeric material and the like. For example, thermosetting resins, such as cross-linked polyethylene (hereinafter, referred to as XLPE.), and thermoplastic resins are often used as the polymeric material.
Some insulated cables laid underwater are used to transmit alternating-current voltage, an inverter voltage (repetitive impulse bipolar voltage), and the like. Such insulated cables can cause a water tree in XLPE and other insulators if used for several years under an alternating repetitive impulse bipolar voltage including inverter surges. A water tree refers to a branch-like insulation degradation phenomenon which occurs in the insulation material of the insulator if an electric field acts on the insulator for a long time in the presence of water. The water tree eventually induces an electrical breakdown.
As a water tree diagnosis technique, an insulation diagnostic system and the like for making an insulation diagnosis of lightening arresters and power cables with a power system stopped have been known (see Patent Literature 1).
The status of occurrence and the like of a water tree need to be figured out in advance in order to select materials for use in the insulators of insulated cables laid underwater and to design their structure. For that purpose, a test for generating, i.e., reproducing a water tree needs to be performed.
Such methods as a water electrode method are used to generate a water tree. FIG. 5 is a sectional elevational view showing a configuration of a conventional water tree testing apparatus. This water electrode method uses a flat test piece 10 of XLPE in which many electrode holes (recesses) 11 are formed. A surface, where the electrode holes 11 are formed, of the test piece is immersed in an application-side aqueous solution 23. The other surface is immersed in a ground-side aqueous solution 33. In such a state, a relatively high electric field is generated near each of the electrode holes 11. A water tree can thus be reproduced in the XLPE around each electrode hole 11.